1. Field of the Invention
This invention relates generally to olefin polymerization catalysts and methods and, more particularly, this invention relates to a method of polymerizing one or more 1-olefins using a catalyst system including a solid catalyst component in combination with a cocatalyst.
2. Description of Related Technology
The use of chain transfer agents such as gaseous hydrogen to control molecular weight of polymers of 1-olefins is well known. In general, the presence of hydrogen during polymerization of 1-olefins effects a reduction in average molecular weight of product polymer. Polymerization catalysts vary in their response and sensitivity to the presence of hydrogen.
In general, melt index (MI) or, in the case of propylene homopolymers and copolymers, melt flow (MF), both as measured by ASTM D-1238, are qualitative indications of polymer molecular weight. In general, molecular weight is inversely related to the MI or MF value of a polymer.
It is desirable to prepare polymers using a catalyst system whereby the MI or MF (and thus, molecular weight) is controllable by selection of hydrogen pressure in the polymerization reactor, and in many cases, it is desirable to produce polymers having high melt flow values. In propylene polymerization, however, it is often necessary to resort to partial degradative processes such as visbreaking of polymers in order to increase MF. Visbreaking may be disadvantageous in that it is by definition an additional, costly step and may result in loss of polymer crystallinity, narrowing of polymer molecular weight distribution, and degradation of other product properties. Thus, it is desirable to prepare polypropylene and other polyolefins having a desired melt flow value directly in a single reactor without the necessity for a degradative visbreaking step.
Further, some prior catalyst systems which allow control of MF and achievement of high MF values do not produce polymer at an acceptably high productivity level.
In at least one known prior catalyst system which allows regulation of polymer molecular weight by means of the use of hydrogen or other regulators, a solid catalyst precursor is modified by contact with a molar excess of a gaseous chlorinating agent (Cl.sub.2 or HCl) in a liquid medium, with the resulting solid carrier being isolated by removal of the liquid phase.
The use of a gaseous chlorinating agent in catalyst preparation may require the use of an excess of the gaseous material to obtain effective gas/liquid/solid contact. The gaseous agent has only limited solubility in the reaction medium, and the use of a gaseous reactant can result in control problems.
Furthermore, such procedures typically include several solid intermediate and product isolation steps, and the resulting catalytic products may exhibit only limited sensitivity to the presence of hydrogen during polymerization, and may not produce product polymer having a high melt flow value.